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Hook a Lottie animation up to ScrollTrigger

If you create an animation in After Effects and export it using Lottie, you can hook it up to the scroll position with this handy function so that as the user scrolls, the animation progresses:

function LottieScrollTrigger(vars) {
  let playhead = { frame: 0 },
    target = gsap.utils.toArray(vars.target)[0],
    speeds = { slow: "+=2000", medium: "+=1000", fast: "+=500" },
    st = {
      trigger: target,
      pin: true,
      start: "top top",
      end: speeds[vars.speed] || "+=1000",
      scrub: 1,
    },
    ctx = gsap.context && gsap.context(),
    animation = lottie.loadAnimation({
      container: target,
      renderer: vars.renderer || "svg",
      loop: false,
      autoplay: false,
      path: vars.path,
      rendererSettings: vars.rendererSettings || {
        preserveAspectRatio: "xMidYMid slice",
      },
    });
  for (let p in vars) {
    // let users override the ScrollTrigger defaults
    st[p] = vars[p];
  }
  animation.addEventListener("DOMLoaded", function () {
    let createTween = function () {
      animation.frameTween = gsap.to(playhead, {
        frame: animation.totalFrames - 1,
        ease: "none",
        onUpdate: () => animation.goToAndStop(playhead.frame, true),
        scrollTrigger: st,
      });
      return () => animation.destroy && animation.destroy();
    };
    ctx && ctx.add ? ctx.add(createTween) : createTween();
    // in case there are any other ScrollTriggers on the page and the loading of this Lottie asset caused layout changes
    ScrollTrigger.sort();
    ScrollTrigger.refresh();
  });
  return animation;
}

Usage​

LottieScrollTrigger({
  target: "#animationWindow",
  path: "https://assets.codepen.io/35984/tapered_hello.json",
  speed: "medium",
  scrub: 2, // seconds it takes for the playhead to "catch up"
  // you can also add ANY ScrollTrigger values here too, like trigger, start, end, onEnter, onLeave, onUpdate, etc. See /docs/v3/Plugins/ScrollTrigger
});

DEMO

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Special thanks to Chris Gannon for his work on a tool that inspired this.

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